The present invention relates generally to optical polarizing devices, and more particularly to an achromatic infrared retarder configured to produce a phase delay of one component of polarized light with respect to the orthogonal component over a large wavelength interval in the infrared without producing beam deviation.
An optical retarder is an optical element designed to retard one linear polarization component with respect to the orthogonal component so as to produce a selected phase shift between the two components. Two types of retarder in common use are the total internal reflection prism and the waveplate. In the total internal reflection prism a specific phase shift occurs between the s and p components of light (linear retardance) on total internal reflection, depending on refractive index (usually wavelength dependent) of the material comprising the prism. The retardance of the prism is independent of thickness and variation of retardance with wavelength is substantially less than that of the waveplate. A common retarder of the prism type is known as a Fresnel rhomb, which has desirably low wavelength dependence but has undesirably large beam offset associated with its use and is undesirably large; for example, a Fresnel rhomb for the infrared made of zinc selenide (ZnSe) having a clear aperture of 0.95 inch has a beam offset of 1.66 inches and a length of 3.65 inches.
The waveplate retarder consists of a plane parallel plate of birefringent material with the crystal axis oriented perpendicularly to the intended propagation direction of an incident light beam. In the waveplate retarder, plate thickness is selected such that plate thickness times the birefringence (difference between the ordinary and extraordinary refractive indices of the plate material) equals an integral number of quarter wavelengths of the incident beam. For an odd integer number, a quarter wave retarder obtains; for the integer equal to one, the plate is very thin and is referred to as a zero order waveplate. Retardance of the zero order waveplate necessarily varies with wavelength unless by coincidence the birefringence is linearly proportional to wavelength. Since this does not occur in practice, the waveplate has only approximately a quarter wave retardance over a small wavelength range. For higher order waveplates (odd integer greater than one) the effective wavelength range for quarter wave retardance is even smaller. The achromatic range of a waveplate retarder can be enlarged with combinations of waveplates of positive and negative birefringent materials at right angles and balancing birefringence variations with wavelength. This method is feasible in the visible, but less birefringence data is available to readily produce designs of a device for the infrared.
The invention solves or substantially reduces in critical importance shortcomings in prior art devices as suggested above by providing an achromatic infrared retarder designed to produce a phase delay of one polarized light component with respect to the orthogonal component and wherein the output and input beams of the retarder are colinear. The invention comprises two identical prisms of selected configuration mounted in confronting relationship on a substrate such that a transmitted beam experiences a total internal reflection in each prism and one total reflection at a reflective surface on the substrate between the prisms. With proper design, the output beam has a quarter wave linear phase delay and changed polarization relative to the input beam. Linearly polarized light can be converted to circularly polarized light and vice-versa because of the quarter wave phase delay produced in the transmitted beam. The invention is useful over a wide wavelength range (viz, 3-14 microns) and has particular application in spectropolarimeters for determining polarization properties of materials, multiple-wavelength laser polarimeters, or ellipsometers for determining thin film thickness and refractive index.
It is therefore a principal object of the invention to provide an improved optical retarder device.
It is another object of the invention to provide an achromatic infrared retarder for producing a phase delay of one component of polarized light with respect to the orthogonal component over a large range of wavelengths.
It is yet a further object of the invention to provide an achromatic infrared retarder wherein the input and output beams are colinear.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.